There are several general categories of pressure measuring devices, e.g., those based on the measurement of the height of a liquid column, those based on the deflection of an elastic element, and those that utilize the piezoelectric response of certain crystals.
Pressure measuring devices based on the deflection of an elastic element operate on the principle that, within the elastic range, the magnitude of the deformation is approximately proportional to the applied pressure. Common embodiments are: Bourdon-tube elements, which indicate pressure by the amount of deflection in an arcuate closed-end tube; bellows elements, which axially deform under pressure; and diaphragm elements, which deflect when subjected to a pressure differential between two faces. In each case, pressure will deform the active element in a known way so that as the pressure increases so does the deformation. Once this deflection curve is calibrated, the pressure may be determined.
To determine the deflection of an elastic element, a strain gage which may be bonded to the surface of the element. When an electrical conductor is bonded to an elongating elastic surface, the conductor is stretched. This increases the length, but decreases the diameter of the conductor, which varies the electrical resistance. This change in resistance therefore provides a measure of the deflection of the element. Once calibration has been performed, the amount of pressure applied to the element may be determined directly from the strain gage signals.
These principles of elastic deformation (i.e., in a flexible diaphragm) and electrical resistance have been used to construct pressure cells that measure the amount of force per unit area that is applied by a solid impinging object, such as ice on the hull of a ship, or soil on an imbedded foundation pile. However, these devices may give inaccurate readings for a variety of reasons.
One source of inaccuracy is the presence of softness or compliance in the flexible diaphragm of the pressure cell. The occurrence of what is called bridging is thought to lead to these erroneously low readings. Bridging may be defined as the tendency of the cell compliance to relieve the pressure that the cell experiences, with additional pressure then being applied immediately adjacent to the periphery of the cell. In other words, the solid impinging object is being supported by the sides of the measurement cell and not the pressure sensing face itself (see FIG. 2). The result is that the measured pressure is artificially reduced.
A second source of inaccuracy, even if bridging is not a problem, is insensitivity of the cell. In conventional designs it may be necessary to make the cell diaphragm relatively stiff so that it may have sufficient strength under a maximum expected pressure. However, as the stiffness increases the sensitivity to relatively low pressures decreases because the diaphragm deflections become smaller. This, of course, narrows the operating range of the device.
Consequently, it is an object of this invention to provide a pressure measurement device that may avoid errors due to compliance and also have greater sensitivity.